Glass mat or roving towards plywood.

[Franziska]

Hi.



Upfront, I know that this is not perfect, but this is all I can get here, so, this is what we will need to use.



We are currently in Tunisia and it's tough to get proper materials for modern wood construction over here. Thankfully I brought plenty of microfibers, microbaloons and epoxy with me.



I wanted to buy some proper 10mm marine ply, but all I could get, was is claimed marine grade ply but with three thin layers and two thicker layers instead of 10thin layers. Not great. So I will need to beef it up with fiber laminates on both sides.



Still, here comes the second problem. They don't have pure woven roving here, only mat backed stuff. Not really what I want to use with epoxy. It's fairly heavy. Square meter weight is 500g woven roving with 300g mat. I'm happy we use it on a flat application.



Question. If I put this onto the plywood, what side should face the plywood from a structural standpoint. Mat or roving side.

If its not so important, I'd prefer the mat outside as it might be smoother.

[Lee Jerry]

For max strength and stiffness, you would want the roving on the outside. It's the continuous fibers of the roving that are the more efficient structural members and you want them separated as much as possible to make the tallest "I-beam." The roving becomes the flanges of the I-beam and the mat/plywood become the web.

[Franziska]

Thanks, that makes sense in the overall picture with regard to the neutral axis.

Still, would the bonding of the roving to the plywood not be better?

[Lee Jerry]

No, the mat actually will provide better bonding too. The random, short fibers in three dimensions provide better "contact" with the plywood.

[Chotu]

I agree with Lee Jerry. Think about the failure modes.

If you bend and bend and bend that until it breaks, if the roving fibers are on the outside, you are going to see a break between the fiber layer and the matte layer upon failure. That never happens. Realistically it would rip off of the plywood probably and take some wood with it.

Also, the matte layer will provide a better bond to the plywood. Uniform

Now picture the failure mode of the reverse. Mat on the outside.

If you bend and bend and bend that, you might see crazing and cracking in the Matt layer because there are no directional fibers in it. Realistically it probably would pull off of the plywood and take plywood with it as well.

So I think the true answer is it’s not going to matter because the failure mode will be the plywood or bond to plywood.

However it might be just ever so slightly stronger assuming the plywood stays intact(which it wouldn’t) , to have the mat on the inside.

Also regarding marine plywood, the layers are important, but also voids. Some of the reason marine plywood is so good is because it’s guaranteed not to have voids. The voids will definitely reduce the strength of whatever you are building as compared to just flipping the mat layer around. So I guess just be sure that is plywood that is free of voids. And who knows what glue they are using so seal it up as if it were not marine plywood

[thinwater]

Mat down for bonding.


I would test a small piece of the mat first. Most mat does NOT play well with epoxy; the bonding resin does not dissolve and become limp in epoxy as it does in polyester resin. This is probably the wrong fabric. Alternatively, not knowing the job requirements, this might be a place for polyester resin.


But test a small snippet first. Does the mat go limp, as it should?

[capt jgw]

The mat layer provides a "squishy" interface between the two different textures of the roving and the surface you apply it to, so the mat goes against the plywood. This is really important in roving with it's heavy coarse weave pattern. If you go the other way the resin has to fill all those gaps which it's not likely to do.

[Franziska]

Thanks. I'm aware that CSM is not right for Epoxy as it lacks the Styrene to break down the binder. But we can't get pure woven here.

Hence the question above.

I have some double diagonal on-board, but I need that for the attachment laminates.

The woven/mat combo goes on flat areas only. I'll put the mat side towards the plywood first and will test a sample before.



Regarding glue, I'm slightly hopeful, they have exterior and marine ply, so I hope marine fairs better. Still will apply two layers of Epoxy wet on wet and a layer of the fiber material on both sides. Once it's cured we'll stick it down with Epoxy.